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(CNN) -- The cow confirmed last week to be infected with mad cow disease was 12 years old and lived its entire life in Texas, according to John Clifford, the U.S. Department of Agriculture's chief veterinarian.

Unable to walk, the animal was selected for testing at a pet food facility in Waco, Texas, he said.

Its carcass was incinerated and no part of the animal was ever used for human or pet food, USDA officials said.

Two cattle related to the infected animal have been identified so far, Clifford said.

The USDA is holding the entire source herd as it continues efforts to test other members of the herd born around the same year as the infected cow, as well as the cow's offspring.

"Experience worldwide has shown us that it is highly unusual to find BSE [bovine spongiform encephalopathy] in more than one animal in a herd or in an affected animal's offspring," Clifford said.

"Nevertheless, all animals of interest will be tested for BSE," Clifford said.

"The safety of our food supply is not in question," he said. "I am very confident that our interlocking safeguards are effective, and this case is evidence of that."

Clifford said his agency is working with the Food and Drug Administration to determine the feed history in the infected cow's herd.

Given the animal's age, Clifford said the USDA believes it was "most likely" infected by consuming feed prior to a 1997 ban on feeding cattle the remains of other similar mammals.

This is the second case of BSE found in the United States. The first case, discovered in December 2003, was in a dairy cow that had been imported from Canada.

BSE is a chronic, degenerative disorder affecting the central nervous system of cattle. It is usually transmitted to cows via contaminated feed and has an incubation period in animals of four to six years.

Since it was first diagnosed in Great Britain in 1986, there have been more than 180,000 cases.

Eating meat from cows infected with the protein has been linked to variant Creutzfeldt-Jakob disease, a fatal brain disorder in humans.